BP caps abandoned cofferdam used on Macondo

BP PLC has capped and plugged an abandoned piece of subsea equipment known as a cofferdam that was believed to be the source of a recent surface oil sheen observed near Mississippi Canyon 252 Block in the Gulf of Mexico off Louisiana.

The cofferdam is an 86-ton, steel container that was lowered over a leaking drill pipe during oil spill response efforts in 2010 following the blowout of the deepwater Macondo well. In 2010, that particular containment dome was unsuccessful, and the equipment was moved and left 500 m from the wellhead.

With US Coast Guard approval, BP deployed remotely operated vehicles on Oct. 23 to install a 750-lb cap over the stovepipe on the cofferdam and secured the cap in place with clamps. ROVs also inserted plugs into four small connection ports on the top and sides of the cofferdam. The operation lasted 26 hr.

Initial visual inspections of the cap and plugs indicated no oil droplets emanating from any of the openings. BP will monitor the sheen by satellite for several days to confirm the cap and plugs are secure.

Last week, a separate ROV survey confirmed that the Macondo well and its two associated relief wells were not leaking. It marked the third time since the Macondo well was permanently sealed in September 2010 that well integrity was confirmed by visual inspection.

The latest survey came after BP and Transocean received a Notice of Federal Interest from the USCG to determine sources of the recent sheen (OGJ Online, Oct. 18, 2012).

The survey focused on the wells, wreckage, and debris related to the Macondo well blowout and subsequent explosion and fire on the Deepwater Horizon semisubmerisible. BP operated Macondo, and Transocean Ltd. owned the Deepwater Horizon, which sank after the incident.

On Sept. 16, BP reported the sheen to the National Response Center. USCG determined it is not feasible to recover the sheen and that the sheen does not pose a risk to the shoreline.

The Federal Onscene Coordinator (FOSC) authorized the containment dome capping procedure in which BP used ROVs from the offshore construction vessel Skandi Neptune.

Representatives from the Coast Guard, Bureau of Ocean Energy Management and Bureau of Safety and Environmental Enforcement and Gulf Coast Incident Management Team's Department of Interior Liaison, along with state representatives from Louisiana, Mississippi, and Florida, observed.

The FOSC directed BP to submit a feasibility plan that considers the next steps toward either removing or remediating the threats of oil posed by the riser pipe and containment dome.